The Independent Police Complaints Commission is investigating a tip-off from a current or ex-London police officer that the London Metropolitan Police's National Domestic Extremism and Disorder Intelligence Unit asked Indian police to use their hacker contacts to break into the private communications of hundreds of British people and groups, from journalists to Greenpeace.
Also targeted were politicians, including Green Party peer Baroness Jenny Jones.
"This will be a complex investigation given the potential involvement of foreign participants.
"We would like to hear from the officer who brought these allegations to light or any other officers or police staff who may be able to provide information of use to the IPCC investigation."
The Met Police said: "The IPCC made the Metropolitan Police Service aware of anonymous allegations concerning the access of personal data and requested the matter referred to them by the MPS. This has been done.
"The MPS is aware that the IPCC is carrying out an independent investigation.